RFR: JDK-8249292: DependOnVariable macro fails on empty value

[Tim Bell]

Erik:

> The DependOnVariable macro fails if the variable is empty. The failure 
> is rather non intuitive as it complains that the vardeps file doesn't 
> exist and is needed for the target. This patch make the macro treat an 
> empty variable value just like any other value. I'm also adding test 
> cases to verify this new behavior.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8249292
> 
> Webrev: http://cr.openjdk.java.net/~erikj/8249292/webrev.01/index.html

Looks good.

Tim